The projection of images using digital light processing methods typically requires the use of a plurality or array of mirrors or micromirrors to focus the light on a screen. As seen in FIG. 1, the array contains a plurality of mirrors 2 that can be tilted to selected angles. Some current examples of the devices that use these mirrors and arrays are rear projection televisions, front projection devices for use in business and cinematic environments, and for marquee displays. FIG. 2 is a picture of a typical Texas Instruments' Digital Micromirror Device (“DMD”) in which a plurality of mirrors is encased in a hermetically sealed housing 4 having a window 6 for the passage of light to and from the mirrors located therein.
FIG. 3 is a schematic of the principle elements of a typical DMD device 10 containing a plurality or array of mirrors. Not shown in FIG. 3 is the housing that surrounds the device, as illustrated at 4 in FIG. 2. The principle elements of the DMD device 10 are the array of mirrors 12, the chrome aperture 14 (gray-filled rectangles), and the window 16 overlying the aperture 14 and array of mirrors 12. When the DMD device 10 is used in, for example, a projection system, incident light 22 (within the solid-line cone 23) from a light source 20 is focused at an angle, for example, an angle in the range of 10-30 degrees (10° to 30°) from the perpendicular to the plane of the window 16 overlying the array of mirrors 12. The incident light 22 passes through the window 16, strikes the array of mirrors 12 and is reflected by the individual mirrors in the array of mirrors 12. Each mirror in the array of mirrors 12 is capable of being tilted at a selected angle determined by the manufacturer. When a mirror in the array of mirrors 12 is tilted so that it is in the ON position, the light is reflected perpendicular to plane of the window 16 as indicated by arrow 30 (within the dot/dash-line cone 31) toward a detector 40. When a mirror in the array of mirrors 12 is tilted so that it is in the OFF position, the light is reflected through window 16 away from detector 40, for example, in the direction indicated by arrow 32 (within the dashed-line cone 33). In either ON or OFF position the light passes through window 16. The ratio of the intensity (“I”) between the ON and OFF positions is defined as the “contrast ratio” (“CR”), where CR=Ion/Ioff.
As illustrated in FIG. 3, the DMD device 10 is illuminated with an f/3.0 cone of light. As illustrated, the incident illuminating white light is coming from a 100-Watt tungsten lamp (or other lamp capable of producing white light) at an angle of 26° to the perpendicular of the window 16. The detector 40 collects light at an f/3.0 cone and is centered above the DMD device 10 as illustrated, normal to the window 16. The DMD device 10 operates in the Ion and Ioff states. In the ON state, Ion is dominated by the DLP window's (16) normal transmission of reflected light from the ON state mirrors behind window 16 toward the detector 40. In the OFF state Ioff is dominated by the residual reflectance from window 16 at 10°-30° incidence angle. Since Ioff is a small value and anti-reflective coating (“ARC”) residual reflectance contributes a large amount to Ioff, it is important that the ARC be designed so that Ioff is minimized. In particular, one needs to design an ARC that has the lowest reflectivity at the selected incoming light incident angle with the lowest polarization dependence over a wide wavelength range. In the above example, one would wish to design an ARC which has the lowest reflectivity at 26° incident angle with the lowest polarization dependence at a wavelength range in 480-640 nm range. Minimizing Ioff reflectance improves, for example, the contrast ratio.
While antireflective coating for windows of DMDs are known, little or no effort has been made to optimize the window 16 coating for angular operation. For example, 3- and 4-layer coating with quarter wavelength thickness are known. In view of the critical nature of anti-reflective coatings toward minimizing Ioff, the development of optimized anti-reflective coating is important to the future development of DMDs and the systems that utilize them. Accordingly, the present invention describes optimized anti-reflective coatings for minimizing Ioff.